The present invention relates to a process for producing plastic moldings from a flowable reaction mixture. In this process, a xe2x80x9cshotxe2x80x9d of flowable reaction mixture is produced by mixing the components of the reaction mixture in the mixing chamber of a mixhead. This shot of reaction mixture is then introduced into a mold where the reaction mixture cures, and the plastic molding is then removed from the mold. The present invention also relates to a device suitable for carrying out this process.
A rotating mold carrier in the form of a so-called rotary table or continuous belt, on which a multiplicity of molds is arranged, is generally used to produce plastic molded articles by processes such as that of the present invention. A series of mold cavities may be filled by placing one of the mold cavities to be filled under the mixhead and/or outlet pipe for an adequate period of time and then moving the mold carrier to cause the filled mold cavity to be removed from under the mixhead and an unfilled mold cavity to be placed under the mixhead and/or outlet pipe. The period of time necessary to fill a mold cavity corresponds substantially to a shot time in each case. When the mixhead is kept stationary, the movement of the mold carrier occurs in a cycle with the mixhead being docked in place for the filling of a mold.
If the mixhead is completely emptied at the end of a shot, ambient air penetrates into the mixing chamber. Because air (i.e. oxygen and nitrogen) has only imperceptibly low solubility in the plastic-forming (e.g., polyol-isocyanate) reaction mixture, the reaction mixture foams when the raw material components are introduced at the beginning of a shot. This foam formed at the beginning of each shot must be collected and discarded to avoid defects in the molding.
According to a proposal of the applicant named in EP-A 1 033 230, foam formation is prevented by providing the mixhead with a closure element and adapting the reactivity of the reaction mixture to the time interval between the end of a shot. In this disclosed process, the mixing chamber outlet is closed by the closure element between the end of the shot and the beginning of the next shot (initiated by opening the closure element) in such a way that the mixing chamber can remain filled between two shots. A process of this kind requires strict observance of the cycle times and/or interval times between the shots to avoid uneconomically long curing times.
It is an object of the present invention to provide a process for the production molded plastic articles in which foam formation at the beginning of a shot including when the mixing chamber is emptied after each shot is prevented.
It is another object of the present invention to provide a device useful for the production of molded plastic articles in which foam formation at the beginning of a shot is prevented.
These and other objects which will be apparent to those skilled in the art are accomplished by removing any ambient air from the mixing chamber before the beginning of each shot.